This invention relates to optical waveguide grating filters and in particular to such filters that exhibit a relatively high degree of isolation between stop and pass bands.
A transmission type filter with a defined pass-band can be constructed in a length of single mode optical waveguide by creating in the waveguide two Bragg gratings providing stop-bands respectively on the long-wavelength side and the short wavelength side of the intended pass-band. These two Bragg gratings may be normally reflective Bragg gratings whose elements extend in parallel planes normal to the waveguide axis, in which case their stop-bands are created by reflection of light within those wavebands reversing the direction of propagation of the guided light. Alternatively, they may be blazed Bragg gratings whose grating elements extend in parallel planes inclined at an oblique angle to the waveguide axis, in which case their stop-bands are created by mode conversion of light propagating in the waveguide from the guided mode into an unguided, and hence radiating, mode or group of modes. By way of example, normally reflective Bragg gratings may be made in optical fibre waveguides by the method described in U.S. Pat. No. 4,725,110, and blazed Bragg gratings by the method described in U.S. Pat. No. 5,042.897. A transmission type filter of this type may be constructed to provide an isolation of about 30 dB between its stop and pass bands. A similar degree of isolation can be obtained in a reflective type filter in single mode waveguide by the use of a single normally reflective Bragg grating.
An example of an optical waveguide Bragg grating filter comprising an optical fibre provided with one or more normally reflective Bragg gratings is to be found for example in U.S. Pat. No. 5,283,686, and the use of blazed gratings for optical filter purposes in for example WO 92/24977.